The maximum mean body weights of rats attained during the course of two-year studies increased by about 20% from 1975 to 1985. Higher body weights will lead to increases in the incidences of mammary tumors, pituitary tumors and possibly other tumors. Modification of diet and feeding procedures may show the growth and lower the maximum body weight attained which in turn may decrease the incidences of spontaneous tumors. The purpose of this study is to determine the feasibility of a 15% protein diet (NTP-88) with restricted feeding from 4 p.m. to 8 a.m. in lowering the maximum body weights of rats in comparison with AD libitum feeding and 24% protein diet (NIH-07). Groups of 25M + 25F F344 rats housed 5/cage by sex are being fed NIH-07 or NTP-88 diet Ad libitum or 4 p.m. to 8 a.m. daily. Body weights and feed consumptions are being determined at one to two-week intervals. Hematology and clinical chemistry will be done at selected intervals and tissues will be collected for histology at the end of the study.